1. Field of the Invention
The present invention relates to a developing apparatus for use with an electrophotographic machine such as a copier or a printer, for agitating and feeding a developer to develop an electrostatic latent image on a photosensitive drum.
2. Description of the Related Art
A developing apparatus for use with an electrophotographic apparatus such as a copier or a printer agitates a developer therein, applies an electric charge of a predetermined polarity to toner, and forms a developer layer on a developing magnet roller with a plurality of fixed magnets mounted therein. The developer layer is carried to an electrostatic latent image on a photosensitive drum, whereupon the toner is applied to the latent image to obtain a visible image.
In recent years, the demand for electrophotographic machines operable at higher speed and over an extended time period has led to the need to maintain the stable performance of developers. Generally, an increase in speed results in an increase in agitating speed inside the developing apparatus, which subjects a developer to increased stress and tends to form aggregates of the developer. The aggregates may adhere to the drum and impair image quality. Further, as a large amount of toner is supplied and agitated at high speed to enable a high-speed operation of the machine, defectively charged toner particles such as insufficiently charged toner particles and those having an opposite polarity to a desired polarity are transported to regions for development. Such situation results in scattering of the toner, image fogging, contamination of machine interiors and smudging of paper. With an increase in the peripheral speed of the developing magnet roller, defective carrier particles of low magnetic permeability may become separated and adhere to the photosensitive drum, thereby lowering image quality.
To date, these problems have been tackled chiefly by improving the toner/carrier compositions and manufacturing conditions. However, such an approach has been short of providing an overall solution to the problems. As a conventional technique of dealing with defectively charged toner particles, Japanese Examined Patent Publication JP-B2 11160 (1990) discloses a developing apparatus with a slit formed in a case bottom thereof, in which low-charged toner particles afloat in the developing apparatus are discharged through the slit, by using a positive pressure in the developing apparatus, to be collected in a toner collecting box.
In the above publication, as noted above, the developing apparatus has a slit formed in the case thereof, and a toner collecting box communicating with the developing apparatus through the slit. Toner particles afloat in the developing apparatus are discharged through the slit to be collected in the toner collecting box. While the publication does not describe a discharging mechanism, the discharging action requires a difference in internal air pressure between the developing apparatus and the collecting box. This conventional technique utilizes an increase in the air pressure in the slit on the developing apparatus side which is caused by revolution of erected parts of the developer formed with a lowermost pair of magnetic poles.
In Japanese Examined Patent Publication JP-B2 11160 (1990) noted above, while utilizing an increase in the air pressure in the slit on the developing apparatus side which is caused by the revolution of erected parts of the developer formed with a lowermost pair of magnetic poles, the air in the developing apparatus and a pocket flows in one direction, resulting in aggregates of the developer and defectively charged toner particles. Copies obtained include image flaws caused by the aggregates, and the machine interiors inevitably become stained with scattering toner.